Internal combustion engines are typically coupled to a starter mechanism for rotating a crankshaft to initiate the combustion cycle and thereby start the engine. The rotation of the crankshaft in this manner is referred to as cranking the engine. In the case of an internal combustion engine for a motor vehicle, the starter mechanism can take the form of an electric starter motor which selectively engages a coupling mechanism to rotate the crankshaft. The starter motor is disengaged once the engine is running to prevent damage.
A hybrid electric vehicle can combine the internal combustion engine with an electric traction machine to generate the motive force for driving the motor vehicle. One approach has been to integrate the electric traction machine into a housing provided in the vehicle driveline. The housing can, for example, be the transmission housing. The electric traction machine can be arranged coaxially with a crankshaft of the internal combustion engine. This arrangement can be referred to as a Crankshaft Integrated Motor Generator (CIMG). A separate electric starter motor can be provided for starting the internal combustion engine; or the CIMG can be used to start the internal combustion engine. The control architectures for both these arrangements will now be described with reference to FIGS. 1 and 2.
A known control apparatus 100 for a hybrid electric vehicle is shown in FIG. 1. The control apparatus 100 is configured to control a CIMG 101 and a starter motor 103 for starting an internal combustion engine. The CIMG 101 is energized by supplying current from a high voltage battery 105 to an inverter 107 which converts the direct current (DC) supply to a three (3) phase alternating current (AC). A separate low voltage battery 109 is provided for energizing the electric motor in the starter motor 103. A unidirectional DC/DC converter 111 is provided to charge the low voltage battery 109 when the CIMG 101 operates as an alternator. High voltage batteries typically operate poorly at low temperatures and at temperatures below −30° C. their capacity may be reduced to such an extent that they are unable to energize the CIMG 101. Due to their chemical composition, low voltage batteries can typically operate at lower temperatures than high voltage batteries meaning that the starter motor 103 can still operate to crank the engine.
Alternate arrangements of the control apparatus 100 to enable the CIMG 101 to function as both a traction motor and a starter motor are illustrated in FIGS. 2A and 2B. A DC/DC converter 111 is provided to allow the low voltage battery 109 to be charged by the CIMG 101 when operating as an alternator. However, a problem may arise at low temperatures since the capacity of the high voltage battery 105 is reduced. At very low operating temperatures, for example below minus 30° C., there may not be sufficient charge in the high voltage battery 105 to energize the combined CIMG 101. If the DC/DC convertor 111 is a unidirectional converter (as shown in FIG. 2A), it is not possible to crank the internal combustion engine at low temperatures since the high voltage battery 105 is unable to energize the CIMG 101. To overcome this limitation, a bidirectional DC/DC converter 111 must be utilised (as shown in FIG. 2B) to enable the low voltage battery 109 to energise the CIMG 101.
The need to provide a starter motor 103 to supplement the electric traction machine 101 is undesirable. However, this arrangement is required with known apparatus to enable the internal combustion engine to be started even when the high voltage battery is unable to energize the CIMG 101, for example due to low operating temperatures. The alternate arrangement in which the CIMG 101 also functions as a starter motor requires the use of a bidirectional DC/DC converter 111. However, the bidirectional DC/DC converter 111 typically increases the cost and complexity of the control apparatus 100.
At least in certain embodiments the present invention sets out to overcome or ameliorate at least some of the shortcomings of the prior art arrangements.